It sounds like you're saying that if you have 2 SIM cards in both slots, the card in slot 2 can't be read, but if you just have a single card in slot 2, then the card can be read. Is this accurate?
What is the serial number of this unit? The serial number might be the MAC address, otherwise a type of electronic S/N such as IMEI, MEID, ESN, so please provide whatever related information you may have about this unit.
What firmware (EOS) version are you running on this unit?